Subgrain boundaries in Antarctic ice quantified by X-ray Laue diffraction

Ilka Weikusat,Sérgio H Faria,Atsushi Miyamoto,Sepp Kipfstuhl,Takeo Hondoh,Nobuhiko Azuma

doi:10.3189/002214311795306628

Abstract

AbstractIce in polar ice sheets undergoes deformation during its flow towards the coast. Deformation and recrystallization microstructures such as subgrain boundaries can be observed and recorded using high-resolution light microscopy of sublimation-edged sample surfaces (microstructure mapping). Subgrain boundaries observed by microstructure mapping reveal characteristic shapes and arrangements. As these arrangements are related to the basal plane orientation, full crystallographic orientation measurements are needed for further characterization and interpretation of the subgrain boundary types. X-ray Laue diffraction measurements validate the sensitivity of different boundary types with sublimation used by microstructure mapping for the classification. X-ray Laue diffraction provides misorientation values of all four crystal axes. Line scans across a subgrain boundary pre-located by microstructure mapping can determine the rotation axis and angle. Together with the orientation of the subgrain boundary this yields information on the dislocation types. Tilt and twist boundaries composed of dislocations lying in the basal plane, and tilt boundaries composed of nonbasal dislocations were found. A statistical analysis shows that nonbasal dislocations play a significant role in the formation of all subgrain boundaries.

Highlights

Changes in the flow of the huge Antarctic ice sheet can significantly influence sea-level rise and are a major cause of uncertainty in the prediction of sea-level evolution (Solomon and others, 2007)
Samples were obtained from the EPICA (European Project for Ice Coring in Antarctica) deep ice core from Dronning Maud Land (EDML) and from one shallow core (B37), drilled between 2003 and 2006 at Kohnen station (758000 S, 08400 E; 2892 m a.s.l.), East Antarctica
Whether all structures are revealed by microstructure mapping can be demonstrated by observing the orientations within ‘clean’ grains without any sublimation substructures visible with the optical microscope (Fig. 3)

Summary

Introduction

Changes in the flow of the huge Antarctic ice sheet can significantly influence sea-level rise and are a major cause of uncertainty in the prediction of sea-level evolution (Solomon and others, 2007). Huybrechts, 2007), a power law typical for dislocation-creep controlled deformation describing ice deformation on a macroscopic scale, but the actual physical deformation or strain-rate controlling processes are not reflected in this law (Alley and others, 2005). Basal glide only provides two independent slip systems, so nonbasal glide has sometimes to be activated, at least locally This is accepted as the rate-limiting process under high-stress conditions leading to a stress exponent n % 3 in Glen’s flow law, but during the low-stress deformation of polar ice sheets these processes are not yet clear (Montagnat and Duval, 2004)

Methods

Results

Discussion

Conclusion